High spatial resolution, range gated, underwater imaging method and apparatus

ABSTRACT

A method and apparatus for imaging objects in turbid mediums, such as water, the invention using a laser pulse, a detector of the reflected laser pulse, and a shutter on the detector. The shutter is kept closed except at the expected return time of the laser pulse from the reflected object to be detected and identified at which time the shutter is opened to permit the detector to receive the reflected laser pulse. Typically, laser pulses with widths of about 100 to about 500 psecs are used. In addition, typical shutter times or gate widths of about 100 to about 500 psecs are also used. Gate widths on the order of 120 psecs are preferred. The detector or GOI camera is shielded from spurious signals, most notably, scattered laser light from the turbid medium.

BACKGROUND OF THE INVENTLON

1. Field of the Invention

This invention relates to a method and apparatus for detecting andidentifying a submerged object in a medium containing particulate mattercausing poor visibility. More specifically, this invention relates to ahigh speed imaging method for detecting and identifying a submergedobject in a murky or turbid medium.

2. Description of the Related Art

Generally, when light is directed at an object, the reflection of thelight allows one to see the object. Suspended particulate matter, suchas dust, mud (in water) and water droplets (fog), however, scatter bothdirect and reflected light greatly reducing visibility. If it reachesthe observer, light which has been forward or backscattered distorts theimage of the object. Only light that has not been either forward orbackscattered and which reaches the observer forms an accurate image ofthe object.

One solution to eliminating the effect of forward scattered and backscattered light has been to gate an optical imager so that only aminimum of scattered light is seen. Unscattered light travels theshortest path from its source to the object and then to the observer.Thus, if one gates an optical imager to open at the precise point intime when unscattered light will arrive at the optical imager and toclose after the unscattered light has arrived at the imager, then mostscattered light will be prevented from obscuring the image of theobject. In effect, by properly gating the optical imager, a clearerimage of the object is formed. This use of a gated optical detector hasbeen discussed in several U.S. patents. For example, see U.S. Pat. Nos.3,682,553; 3,902,803; 4,862,257; 4,920,412; 4,967,270; and 5,013,917.Additionally, in U.S. Pat. No. 3,151,268, gate widths or exposure timesof 3 nanoseconds or less are used. In U.S. Pat. No. 3,467,773, gatewidths or exposure times of 20 nanoseconds are used. In U.S. Pat. No.3,499,110, gate widths or exposure times of 10 nanoseconds are used.U.S. Pat. No. 3,527,881 suggests that the problems associated withunderwater exploration are due to "backscatter," "forward scatter," and"attenuation." See column 2, lines 27, 44 and 51, respectively. U.S.Pat. No. 3,856,988 provides for an imaging system for environments wherelight scattering reduces image quality.

Turbid water inherently backscatters light. In underwater explorationsystems, the intrinsic absorption and scattering properties of the waterlimit the usefulness of prior art systems. In addition to using shortexposures or gate widths, the use of short pulsed sources of light alsoreduces the amount of scattered light entering a gated optical imagerwhile it is gated open. In water, the source of illumination isgenerally a green or bluegreen laser. If the illuminating source is "on"continuously, there is a constant amount of scattered light present inthe medium. In such an environment, where there is an abundance ofscattered light present, the use of a gated optical imager GOI isdefeated because the amount of scattered light entering the GOI cannotbe greatly reduced. The effect of using a continuous source of lightrather than a pulsed source is to introduce into the system an increasedlevel of background noise or background scatter that will necessarilyenter the GOI whenever it is gated open irrespective of the duration ofthe gate width. This scattered light, or backscatter, reduces thecontrast between objects in the scene. Thus, underwater imaging systemsmost often operate in a pulse gated mode. This method of operation hasbeen referred to as a range-gating technique.

It is suggested that the shorter the pulse width of the light source andthe shorter the gate width of the gated optical detector, the better theclarity of the image obtained using a range-gating technique. However,given a short pulse width and a short gate width, little else has beendone to improve the image quality of photographs of an object submergedin a turbid medium such as water. There is a need for a method forimproving the image quality of an object in a turbid medium other thanby just reducing the pulse width and the gate width.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to obtain an image ofan object in a turbid medium with better resolution and contrast claritythan heretofore possible.

It is another object of the present invention to obtain an image of anobject in a turbid medium at a greater depth or distance with betterresolution and contrast clarity than heretofore possible.

These and other objects are accomplished by using a range-gatingtechnique wherein the gate width used is shorter than heretofore usedand by timing the gate width to open during the rise portion and/or thepeak portion of the reflected laser pulse from the object to be imaged.

These and other objects and advantages of the invention may be readilyascertained by referring to the following detailed description andexamples of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and several of theaccompanying advantages thereof will be readily obtained by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a diagrammatic view of a system according to the presentinvention.

FIG. 2 is another diagrammatic view of another system according to thepresent invention.

FIG. 3 made up of FIGS. 3a, 3b, and 3c is a timing diagram showing thetime relationship of a single pulse during the operation of the systemof FIG. 1.

FIG. 4 contains photographs taken according to the present inventiondepicted in FIG. 1 and corresponding to time regions a, b, c and dmarked in FIG. 3.

FIG. 5 contains another image taken, according to the present inventioncorresponding to a gate width of 120 picoseconds during the peak portionof the reflected laser pulse, using a gated optical imager GOI and acharge coupled camera (CCD). The pulse width of the source laser beampulse was 500 picoseconds at an energy of 60 millijoules (mJ). Thetarget distance was ˜15 feet from the window via the reflecting mirrorlocated in the turbid water tank, similar to the tank used in FIG. 1.The attenuation length of the turbid water was 3.4.

FIG. 6 contains the dimensions of the target 46 used in FIG. 1 andimages of which are depicted in FIGS. 4 and 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description of the preferred embodiment isprovided to aid those skilled in the art in practicing the presentinvention. However, the following detailed description of the preferredembodiment should not be construed to unduly limit the presentinvention. Variations and modifications in the embodiments discussed maybe made by those of ordinary skDll in the art without departing from thescope of the present inventive discovery.

In range-gating, a very short, laser pulse illuminates the object, andthe camera shutter is time delayed and only opens for a very shortperiod of time when the reflected light returns from the object. Theidea is to select the unscattered, reflected light from the object thatarrives at the camera, blocking out the scattered photons, which followa shorter or longer path, and which blur the image and reduce the imagecontrast. The GOI camera only views a layer of the medium, such aswater, the thickness of which corresponds to the GOI camera gate width.In other words, the thickness of the layer viewed is equal to the speedof the reflected laser pulse in the medium containing the object, suchas turbid water, multiplied by the gate width, or the time the GOIcamera shutter is kept open. Range-gated imaging has been known but theimprovement here is to shorten both the laser pulse width and the gatewidth and to time the gate width to open and close within a time periodcorresponding to the rise portion and/or peak portion of the reflectedlight pulse, for example, time regions b and c as shown in FIG. 3b.

Referring to FIG. 3, a diagrammatic view of the time relationship of asingle laser pulse during operation of the system of FIG. 1 is shownwherein FIG. 3a is a time versus intensity plot of an exemplary laserpulse emitted by a pulsed laser source. According to FIG. 3a, a laserpulse is emitted at time to and has a duration between about 100 toabout 500 picoseconds (psec). Typically, a pulse width of 300 to 500picoseconds is used. Particularly in FIG. 3a, an exemplary pulse widthof about 500 picoseconds (0.5 nanoseconds) is depicted. This pulsecorresponds to laser beam pulses 16 and 106 in FIGS. 1 and 2,respectively. FIG. 3b is a time versus intensity plot of an exemplarylaser pulse that has been reflected by objects 46 and 170 in FIGS. 1 and2, respectively. Time regions a, b, c and d of FIG. 3b correspond to atime period of about 100 to about 500 picoseconds, respectively. In FIG.3b, the time regions a, b, c and d correspond to an exemplary timeperiod of about 250 picoseconds, respectively. Gatewidths a, b, c and dare all of equal duration. Typically, gate widths range from about 100to about 500 picoseconds, preferably from about 100 to about 300picoseconds, more preferably from about 100 to about 150 picoseconds andmost preferably from about 100 to about 125 picoseconds. In addition,each of the time periods a, b, c and d in the exemplary plot of FIG. 3bbegin at a time about t_(r), about t_(r), +0.25 nanoseconds (nsec orns), about t_(r) +0.50 nanoseconds and about t_(r) +0.75 nanoseconds,respectively. During time regions or gate widths a, b, c and d the gatedoptical imager camera has its shutter in the open position. Theresolution photographs of target 46, depicted in FIGS. 1 and 6, takenduring time regions a, b, c and d (see FIG. 3) are depicted in FIG. 4.

The principle idea underlying the present invention is to use a gatewidth selected to open the GOI camera shutter during time regions on therise portion and/or the peak portion of the reflected pulse travelingfrom the object to the GOI camera. The rise portion of the reflectedpulse corresponds to the portion of the curve shown in FIG. 3b where thecurve has a positive slope. The peak portion of the reflected pulsecorresponds to the portion of the curve shown in FIG. 3b where the slopeof the curve is either zero or non-existent at maximum intensity andabout ±200 psec or less on either side of the maximum intensity of thereflected pulse. It is apparent from FIG. 4 that even though the gatewidths of time regions a, b, c and d were of equal duration, theresolution seen in the photographs (see FIGS. 4) of the target 46, asdepicted in FIGS. 1 and 6, has varying degrees of clarity. Time region ayielded no image at all because the integrated intensity of thereflected pulse was below a detectable level. See FIG. 4a. Time regionsb and c yielded better results than time region d. See FIGS. 4b, 4c and4d, respectively. Though not previously apparent, time regions b and cyielded surprisingly better results than region d. It was expected thatso long as the gate width was the same, little difference would havebeen visible in the resolution seen on a photograph taken during timeregions a, b, c and d.

Referring to FIG. 1, the present inventive method relates to apulse-gating technique wherein a laser source 10 emitting a laser beampulse 16 suitable for use in turbid water 18 is used in conjunction witha gated optical imager 22, timing electronics 24, gating electronics 26,photodiode 30, mirrors 32, 34, 36 and 38, imaging lens 40, narrowbandfilter 42, water tank 44, and target 46.

EXAMPLE 1

The laser source 10 used in the present invention and depicted in FIG. 1was a Nd:YAG laser with a triple-pass ring amplifier. The amplifier hada small signal gain in excess of 8×10³ and was used in conjunction witha low power, Q-switched, actively mode-locked oscillator and anelectro-optic switch-out system to amplify single 60-400 mJ pulses of500 psec duration in an approximately 2 times diffraction limited pulse.This pulse was then frequency doubled in a KD*P type II crystal with 42%net energy conversion efficiency. The output pulse was synchronized toan external master trigger with a jitter of +/-6 nsec. Additionaldetails of the laser source 10 used are given in Jackel, S. and Burris,R., Multiple-Pass Ring Amplifier for Feedback-Free Amplification ofShort Pulses with High Gain and Energy Extraction Efficiency, OSAProceedings on Advanced Solid-State Lasers, (Chase & Pinto, Eds., 1992)Vol. 13, 104-108, incorporated herein by reference. Additionally, thefrequency doubled Nd:YAG laser had an exemplary pulse width of about 0.5nsec at an exemplary wavelength of about 532 nanometers (nm), which isapproximately the wavelength selected for maximum transmission incoastal waters. Wavelengths for different mediums, other than coastalwater, may vary. The mode-locked Nd:YAG laser frequency-doubled to givea wavelength of 532 nm was designed to give an output pulse energy of upto 160 mJ and a pulse duration of ˜500 psec. For the GOI photographs andCCD images (see FIGS. 4 and 5, respectively) of the target 46, depictedin FIGS. 1 and 6, a pulse energy of 60 mJ was used to avoid stressingthe laser. The laser output beam 16 was ˜0.5 cm in diameter and highlycollimated; thus, to illuminate a large area, it was necessary toincrease the beam divergence using a combination of negative andpositive lenses (not shown in FIG. 1). The lenses were arranged toproduce a laser spot of about 1 foot in diameter at the target 46, whichwas at a distance of about 15 feet in water.

A beam splitter (not shown in FIG. 1) at the output end of the laserdiverted a very small amount of energy from each laser pulse to send tothe photodiode 30 a very low jitter timing pulse for the timingelectronics 24. A pulse was then sent to a delay box (not shown in FIG.1), for suitable delay, and then to the gating electronics 26 to gateopen the GOI camera 22 at the time the reflected laser pulse arrived atthe GOI camera 22 from the target 46. The GOI camera 22 utilized astandard 18-mm microchannelplate (MCP) wafer intensifier, gated by theapplication of a short gate pulse to the cathode with respect to the MCPinput. The gate pulse was applied via a capacitive coupling scheme whichovercame finite cathode resistivity and large capacitive loading. Thisscheme resulted in the removal of irising and allowed the GOI camera 22to be gated with gate widths from about 100 psec to about 5 nsec.

The GOI camera 22 used was a Grant Applied Physics, Inc., Gated OpticalImager (GOI) which gave a single frame with a minimum exposure time ofabout 120 psec full width half maximum (FWEM) over a full 18-mm diametercathode aperture. The super-fast gating speeds were obtained with asolid-state electronic pulser with a jitter of less than 50 psec and atrigger delay of ˜14 nsec. The resolution was typically 10 line pairsper millimeter. A Polaroid back was part of the GOI camera 22, andPolaroid Type 612 (ASA 20000) film was used. Additional details of thecamera used are given by E.A. Mclean et al., in Nanosecond FramingPhotography for Laser-Produced Interstreaming Plasmas, SPIE VOL. 981HIGH SPEED PHOTOGRAPHY, VIDEOGRAPHY, AND PHOTONICS VI (1988) at 186-192,incorporated herein by reference.

The target 46 was located in the large water tank 44. The large watertank 44 had the dimensions of 75'×6'×4' in which the water was madeturbid by the addition of Maalox (aluminum hydroxide and magnesiumhydroxide) to simulate turbid coastal waters. With the target 46 about15 feet from the window 48, the water in tank 44 had an attenuationlength of about 6.4. Uniformity of the Maalox was maintained bycirculating the water while image data was collected by the GOI camera22.

The target used is depicted in FIG. 6. As depicted in FIG. 6, a flatdisk resolution chart with alternate white and black stripes painted onit was used. The width of the stripes varied from 1/4" to 1" and thedisk was 10 inches in diameter. The arrangement of the stripes isdepicted in FIG. 6.

The photodiode 30 used was a Fairchild FND-100. The timing generator wasrun on a single pulse basis, and the gating electronics allowed a gatewidth of about 100 psec to about 500 psec, as required. In particular, agate width of 250 psec was used for the images captured on Polaroid Type612 (ASA 20000) film seen in FIG. 4 and a gate width of 120 psec wasused for the image captured via a CCD camera on a video monitor depictedin FIG. 5.

The laser pulse 16 at 532 nm emitted by the laser source 10 wasreflected by mirrors 32, 34 and 38 to direct the laser pulse 16 attarget 46. The water tank was completely filled with turbid water andhad a distance of about 13 feet from mirror 38 to the target 46. Thereflected laser pulse from target 46 was again reflected by mirror 38back out through Plexiglass window 48 to mirror 36 which in turnreflected the pulse through imaging lens 40 and narrow band filter 42 tothe GOI camera 22. The imaging lens 40 used was a F/6 lens about 5 cm indiameter. The narrow band filter 42 had a narrow bandwidth of about 2 nmwhich was used as an interference filter which peaked at 532 nm. Thisnarrow band filter 42 effectively eliminated much of the scatteredsunlight which is present during daytime operation. The gate width wasabout 250 psec and the times used for the photographic images taken anddepicted in FIGS. 4a, 4b, 4c and 4d were 0 nsec, 0.25 nsec, 0.50 nsecand 0.75 nsec, respectively, after t_(r).

The following example relates to a proposed system for implementing thepresent invention.

EXAMPLE 2

Referring to FIG. 2, connect timing generator 100, to both a pulsedlaser 102, and a charge couple device camera (CCD camera) 104. Operatethe timing generator 100 between about 10-20 Hz. Generate a triggerpulse from the timing generator 100 and generate a laser pulse 106 fromthe Nd:YAG laser 102. Double the frequency of the laser pulse from laser102 so that laser pulse 106 operates at about 532 nm for use in a watermedium and has a pulse width of about 500 psec or less. Direct the laserpulse 106 towards beamsplitter 108 wherein part of the laser beam pulse106a is diverted towards beam expanding lenses 110 and 112 and a smallerpart of the laser beam pulse 106b) is directed towards an attenuatingfilter 116 and a diffuser 114. The beamsplitter 108 is a standard glassmicroscope slide oriented at an angle sufficient to split the laserpulse 106 into 106a and 106b as depicted in FIG. 2. Pass the laser beam106b through the attenuating filter 116 and diffuser 114, such that thebeam 106b encounters photodiode 118. A suitable attenuating filter 116is a Kodak Wratten Neutral Density filter such as the "6.0 ND" filterwhich is properly calibrated for use at 532 nm. The diffuser 114 is a 2πradian diffuser.

Direct the output of photodiode 118 to a photodiode bias supply 120 theoutput of which is directed to a trigger delay unit 122. The photodiodebias supply 120 is a 9 volt bias box. Direct the output of the triggerdelay unit 122 to the GOI pulser 124 which has an attached speed-upmodule 126. For example, a suitable trigger delay unit 122 ismanufactured by Hamamatsu Inc., Model C 1097. Additional details of asuitable GOI pulser are given by EA. Mclean et al., in NanosecondFraming Photography for Laser-Produced Interstreaming Plasmas, SPIE VOL.981 HIGH SPEED PHOTOGRAPHY, VIDEOGRAPHY, AND PHOTONICS VI (1988) at186-192, incorporated herein by reference.

Direct the output from the GOI pulser 124 to the GOI (gated opticalimager) 130 which has the GOI power supply 128 connected to the GOI 130.A suitable GOI power supply 128, the GOI pulser 124, the speed up module126 and the GOI 130 are all manufactured by Grant Applied Physics Inc.,Model GOI/18.

Optically couple the GOI 130 to a charge coupled device (CCD) camera 104via a fiber optics taper 132. Channel the image collected at the GOI 130to the CCD camera 104 through a fiber optics taper 132. A suitable fiberoptic taper 132 is manufactured by Galileo Electro-Optics Corp. whichhas a coupling efficiency of ˜50% for a 2:1 fiber optics taper. Use thefiber optic taper 132 to reduce the exemplary 18 mm output of the GOIcamera 130 to facilitate input to the smaller input of the CCD camera104. Direct the output of the CCD camera 104 to a computer processor 140which is connected to a video monitor 142. For example, a suitable CCDcamera 104 is manufactured by Photometrics Ltd., Model CH 250, Star 1having 384×576 pixels, 12 bit resolution and a dynamic range of 4096. Inaddition, an exemplary computer such as a COMPAQ CTS Model No. 2424CTSCMP1 and an exemplary video monitor such as Dell Model VC2 aresuitable for use in the proposed system of FIG. 2.

As mentioned earlier, pass laser beam pulse 106a through the beamexpanding lenses 110 and 112 and then through an exemplary Plexiglasswindow 150a located at the bottom of, for example, a boat where thewindow is in direct contact with the surface of the turbid water 160. Asuitable exemplary beam expanding lens 110 is a negative lens with anexemplary focal length of about 25 cm and a suitable exemplary beamexpanding lens 112 is a positive lens with an exemplary focal length ofabout 25 cm. Using the beam expanding lenses 110 and 112, pass laserbeam 106a through window 150a into the turbid water medium 160.

Pass laser beam pulse 106a through the water 160a towards the object 170located at a depth 172 from the surface of the turbid water 160. Thelaser beam pulse 106a impinges on the object 170 and is reflected backtowards the surface of the water 160 as reflected laser beam pulse 106c.Reflected laser beam pulse 106c passes from the object 170 towards thesurface of the water 160. Pass laser beam pulse 106c back throughanother exemplary Plexiglass window 150b in contact with the surface ofthe water 160.

Pass laser beam pulse 106c through window 15Ob, imaging lens 152 andband pass filters 154. Gate the GOI 130 open when unscattered reflectedlaser beam pulse 106c reaches the GOI 130 and thereby creates an imageof object 170. Gate the GOI 130 closed after the reflected beam 106c iscaptured by the GOI 130 to minimize forward and back scattered lightfrom entering the GOI 130 and distorting the image of the object 170.

What is claimed is:
 1. A method of detecting and imaging an object in aturbid medium which is at least partially transmitting to lightcomprising the steps of:illuminating said object with a pulse of laserlight having a pulse width of about 100 picoseconds to about 500picoseconds whereby said object reflects said pulse of laser light, saidreflected pulse having a rise portion, a peak portion and a decayportion; detecting, with a gated optical detecting means for detectingsaid reflected pulse, said rise portion, or said peak portion of saidreflected pulse over a period of time no longer than the combinedduration of said rise portion and said peak portion after a delay timecorresponding to a shortest possible propagation time of said pulse oflaser light traveling from a source to said object and reflected to saidgated optical detecting means.
 2. The method of claim 1, wherein saidpeak portion of reflected pulse has a width of about 400 picoseconds andis centered about a point at which said reflected pulse is at a maximumsignal intensity.
 3. The method of claim 1, wherein said laser pulse hasa width of about 300 to about 500 picoseconds.
 4. The method of claim 1,wherein said gated optical detecting means is gated to open for a periodfrom about 100 to about 500 picoseconds.
 5. The method of claim 1,wherein said gated optical detecting means is gated to open for a periodfrom about 100 to about 300 picoseconds.
 6. The method of claim 1,wherein said gated optical detecting means is gated to open for a periodfrom about 100 to about 200 picoseconds.
 7. The method of claim 1,wherein said gated optical detecting means is gated to open for a periodfrom about 100 to about 150 picoseconds.
 8. The method of claim 1,wherein said gated optical detecting means is gated to open for a periodfrom about 100 to about 125 picoseconds.
 9. The method of claim 1,wherein said turbid medium is turbid water.
 10. The method of claim 9,wherein said laser pulse has a wavelength of 532 nanometers.
 11. Adevice for detecting and imaging an object in a turbid medium which isat least partially transmitting to light comprising:illuminating meansfor illuminating said object with a pulse of laser light having a pulsewidth of about 100 picoseconds to about 500 picoseconds whereby saidobject reflects said pulse of laser light, said reflected pulse having arise portion, a peak portion and a decay portion; and detecting meansfor detecting said object with a gated optical detecting means fordetecting said reflected pulse, said rise portion, or said peak portionof said reflected pulse over a period of time no longer than thecombined duration of said rise portion and said peak portion after adelay time corresponding to a shortest possible propagation time of saidpulse of laser light traveling from a source to said object andreflected to said gated optical detecting means.